Re: Community Poll: What do you think about qi supporting n-ary operators?

Frank Hein

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Von meinem iPhone gesendet
> Am 08.08.2016 um 07:33 schrieb Joel de Guzman <[email protected]>:
>
>> On 08/08/2016 8:24 AM, Frank Hein wrote:
>>
>> Am 08.08.2016 um 02:05 schrieb Frank Hein <[email protected]
>> <mailto:[email protected]>>:
>>
>>>
>>>
>>> Von meinem iPad gesendet
>>>
>>>> Am 07.08.2016 um 22:40 schrieb Joel de Guzman <[email protected] <mailto:[email protected]>>:
>>>>
>>>>> On 07/08/2016 12:29 PM, Frank Hein wrote:
>>>>> I reworked the description of the Pull Request on Github. (
>>>>> https://github.com/boostorg/spirit/pull/200)
>>>>>
>>>>> For your convenience all the information from the initial post are now
>>>>> part of the description. I even added a bunch of new arguments and
>>>>> considerations for your review.
>>>>>
>>>>> I would highly appreciate any feedback. Did I miss something important? Do
>>>>> you want that feature? Or is it not needed or superfluos?
>>>>> Is my writing understandable (I am obviously not a native speaker ;). Is
>>>>> there something you want me to explain in more detail?
>>>>>
>>>>> Is there anybody out there?
>>>>
>>>> How about we start with a use-case? What would this facility provide that
>>>> the current code does not, or does so inelegantly?
>>>
>>> Points 1, 3, 5, 8 and 9 of my argument list describe what capabilities qi currently
>>> does not provide.
>>>
>>> This PR enables the implementation of a new class of parsers, n-ary operators.
>>> It does not introduce any particular parser of that class. So talking about particular
>>> use cases is somewhat difficult.
>>>
>>> Anyway, I provided a link to sehe's implementation of an n-ary operator for X3,
>>> longest_match. http://coliru.stacked-crooked.com/a/cb13c1d357cdc464. This can't be done
>>> with qi currently.
>>>
>>> In other words: Implement a parser which works like qi alternative. It takes a variadic
>>> number of alternative parsers and delivers a boost::variant made from these parser's
>>> attribute types. Different to qi alternative this parser should not return the first
>>> match. It should return that match, where parsing consumed most of the input,
>>> independent of the ordering of the alternative parsers.
>>>
>>> You can not solve the whole class of problems which this particular exercise represents
>>> with qi currently. With the PR you can.
>>
>> It is not completely true, that something working like that was completely impossible. But
>> a solution would need a binary operator, which would persistently store info about it's
>> operands. Maybe with an additional directive which takes the operator chain as an argument
>> (a wrapper to know where the operator chain begins and ends) which has also access to the
>> persistent context store, this could be done somehow.
>>
>> But that would introduce a directive and an operator which must be used together always.
>> Not very clean.
>>
>>
>>
>>> Please let us not discuss if this particular example parser is a good idea or not. That
>>> would be a different topic.
>
> I'm not sure about that. I always preferred:
>
>    longest[a | b | c]
>
That would would not work with the qi alternative operator, because it returns the first match. You'd have to implement a new one working together with longest.

And isn't

    longest[a | b | c]

somewhat sophisticated to understand compared to

   longest(a, b, c)?

What does the operator in your example do what's not already expected to be done by longest?

Is max[a+b+c+d] ( or max[a|b|c|d] if you like) a better expression then max(a,b,c,d), if you want the maximum of these four values? Just to illustrate.

With the PR the implementation of longest(a,b,c) is a just a few lines of code (beyond the usual boilerplate to make that a function parser).

> which IMO, is cleaner. Granted, "cleaner" is a subjective term. Anyway, use-cases
> always make concepts clearer. It is difficult to discuss any subject in purely
> abstract terms.
>
> Regards,
> --
> Joel de Guzman
> http://www.ciere.com
> http://boost-spirit.com
> http://www.cycfi.com/
>
>
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